Method of and apparatus for measuring the power distribution in nuclear reactor cores

Abstract

The invention disclosed is the method of exact calibration of gamma ray detectors called gamma thermometers prior to acceptance for installation into a nuclear reactor core. This exact calibration increases the accuracy of determining the power distribution in the nuclear reactor core. The calibration by electric resistance heating of the gamma thermometer consists of applying an electric current along the controlled heat path of the gamma thermometer and then measuring the temperature difference along this controlled heat path as a function of the amount of power generated by the electric resistance heating. Then, after the gamma thermometer is installed into the nuclear reactor core and the reactor core is operating at power producing conditions, the gamma ray heating of the detector produces a temperature difference along the controlled heat path. With the knowledge of this temperature difference, the calibration characteristic determined by the prior electric resistance heating is employed to accurately determine the local rate of gamma ray heating. The accurate measurement of the gamma heating rate at each location of a set of locations throughout the nuclear reactor core is the basis for accurately determining the power distribution within the nuclear reactor core.

I claim to have invented: 
     
       1. The method of monitoring elongated fuel elements, which emit gamma rays, of a nuclear reactor core, comprising: providing an elongated instrument element and locating said instrument element in its monitoring position which is adjacent to, and parallel to, said elongated fuel elements,   passing a cooling fluid adjacent said elongated instrument element,   at least partially isolating a zone of said instrument element from said cooling fluid so that said zone rises in temperature as a result of impingement of said gamma rays thereon,   providing a zone whose temperature depends upon the temperature of said cooling fluid to a greater degree than the temperature of said first-named zone depends upon the temperature of said cooling fluid,   measuring the temperature difference between said zones, and   calibrating the elongated instrument element, by (1) heating the instrument element with measured electrical power while it is in said monitoring position and the temperature difference between said zones is being measured, and (2) analyzing the results of the last-named measurement.   
     
     
       2. The method as defined in claim 1 in which the measured electrical power is varied during the calibration stop and the temperature difference between said zones is measured for different values of said power. 
     
     
       3. The method as defined in claim 2 in which the calibration step is performed while the nuclear reactor core is on. 
     
     
       4. The method of monitoring elongated fuel elements of a nuclear reactor core comprising: providing an elongated measuring instrument and locating it adjacent, and parallel, to said elongated fuel elements,   cooling the elongated measuring instrument with reactor cooling fluid,   changing the direction of flow of the cooling fluid to divert the cooling fluid away from the elongated measuring instrument at a plurality of spaced limited portions of the length of said elongated measuring instrument while passing the cooling fluid in contact with the elongated measuring instrument at other portions of said elongated measuring instrument while the reactor is in operation, and   separately making a plurality of temperature difference measurements, with each such measurement being between one of said portions about which the cooling fluid was diverted and one of said portions which is contacted by the cooling fluid.   
     
     
       5. In the method of claim 4: insulating the portions of the measuring instrument about which the cooling fluid is diverted, from the cooling fluid, by maintaining an evacuated or gaseous space between said last-named portions and the cooling fluid.   
     
     
       6. In the method of claim 5: providing a sealed chamber to divert said cooling fluid.   
     
     
       7. In apparatus for monitoring elongated fuel elements that emit gamma rays, of a nuclear reactor core: an elongated measuring instrument having a plurality of thermocouples with their hot junctions spaced apart along the instrument and respectively responsive to the temperatures of the portions of the instrument where the hot junctions are located,   each said hot junction being spaced apart from its complementary cold junction,   means for guiding said cooling fluid along said elongated measuring instrument to cool the instrument where the cooling fluid contacts it while the reactor is in operation,   each said cold junction being responsive to the temperature of a part of the instrument cooled by said cooling fluid,   insulating means surrounding a portion of the measuring instrument where each hot junction is located to insulate such portion of the instrument from the cooling fluid; whereby the gamma rays may heat the portion of the instrument to which the hot junctions are responsive;   said insulating means including means for diverting said cooling fluid away from said measuring instrument at the places where said hot junctions are located.   
     
     
       8. Apparatus as defined in claim 7 in which each said insulating means comprises a sealed chamber surrounding the portion of the instrument where a hot junction is located. 
     
     
       9. Apparatus as defined in claim 8 in which a partial vacuum exists in said sealed chamber. 
     
     
       10. Apparatus as defined in claim 8 in which said sealed chamber contains a gas. 
     
     
       11. Apparatus as defined in claim 8 in which said sealed chamber is welded to said measuring instrument. 
     
     
       12. Apparatus as defined in claim 8 in which said sealed chamber is composed of material impervious to fluid flow therethrough. 
     
     
       13. The method of monitoring the elongated fuel elements of a nuclear reactor core which elements emit gamma rays, comprising: providing an elongated measuring instrument constructed to be heated to a greater extent by said gamma rays at certain zones along its length than at least, one other zone along its length,   calibrating said instrument which electrical resistance heating that provides heating at each zone of the same magnitude to that of said gamma rays while the reactor is in operation,   locating said instrument adjacent, and parallel, to said elongated fuel elements, and   making measurements, based on the heating contributed by said gamma rays to at least some of said zones, which in view of said calibrations enables a determination of the power outputs of the fuel elements at different positions along their lengths.   
     
     
       14. The method of claim 13 in which the calibrating step precedes the step of locating the measuring instrument adjacent to the elongated fuel elements. 
     
     
       15. The method of claim 13 in which the calibrating step is performed after the step of locating the measuring instrument adjacent to the elongated fuel elements. 
     
     
       16. The method of claim 13 comprising checking the calibration of the instrument by again calibrating it after turning on the power at the elongated fuel elements. 
     
     
       17. The method of monitoring elongated fuel elements, which emit gamma rays, of a nuclear reactor core, comprising: (a) providing a flow path for the flow of a cooling fluid to be used for calibration purposes, and passing said cooling fluid along said flow path for calibration purposes,   (b) providing an elongated instrument element including electrical conducting material having first and second zones,   (c) locating said instrument in said flow path and exposing it to said fluid so that the temperature of the second zone depends on said temperature and rate of flow of said cooling fluid more than the temperature of the first zone depends on the temperature and rate of flow of said cooling fluid,   (d) passing an electrical current, for calibration purposes, through said electrical conducting material to supply heat to both of said zones with the first zone rising in temperature more than the second zone due to cooling effect of said cooling fluid on said second zone,   (e) measuring the temperature difference between said first and second zones to calibrate the instrument,   (f) placing the instrument parallel to and adjacent said elongated fuel elements,   (g) passing a cooling fluid past the instrument while it is adjacent said elongated fuel elements,   (h) the step of passing a cooling fluid past the instrument for calibration purposes as aforesaid involving fluid cooling conditions substantially identical to those characterizing the cooling fluid that is passed by the instrument while it is adjacent to the elongated fuel elements, and   (i) measuring the temperature difference between said two zones while the instrument is adjacent the elongated fuel elements with cooling fluid flowing past the same and without said electrical current flowing, whereby in view of the previous calibration of the instrument with said flow of current the output of the elongated fuel elements may be determined.   
     
     
       18. The method of claim 17 in which during step (i), the first zone rises in temperature above the second zone by an amount related to the output of the elongated fuel elements, and in which water is selected as the cooling fluid. 
     
     
       19. The method of claim 18 in which the cooling fluid is in such good thermal contact with the second zone that the second zone remains at a temperature substantially the same as that of the cooling fluid with the first zone rising to a higher temperature both during calibration as well as during operation adjacent the elongated fuel elements. 
     
     
       20. The method of monitoring elongated fuel elements as defined in claim 17 in which steps (a) to (e) inclusive are performed with said instrument positioned in a remote location with reference to said elongated fuel elements so that those elements do not supply substantial gamma rays to the instrument and so that the instrument is calibrated while the only heat supplied to the instrument during calibration results from said electrical current, and performing steps (f), (g) and (i) after the instrument has been calibrated in said remote location.   
     
     
       21. The method of monitoring elongated fuel elements as defined in claim 20 in which the instrument is calibrated as set forth in said steps (a) to (e) using a first flow path for the cooling fluid, and the elongated fuel elements are monitored as set forth in steps (f), (g) and (i) using a second flow path for the cooling fluid which second path is adjacent said elongated fuel elements and is remote from the first flow path. 
     
     
       22. The method of monitoring elongated fuel elements as defined in claim 17 in which step (f) is performed before the instrument is calibrated, and in which: the nuclear reactor core is shut down before the instrument is calibrated and in which the instrument is calibrated as called for by said steps (a) to (e) while the instrument is adjacent the elongated fuel elements and the nuclear reactor core is shut down.   
     
     
       23. The method of monitoring elongated fuel elements as recited in claim 22 in which the same flow path for the flow of the cooling fluid is used during said calibration steps (a) to (e) inclusive as is used for the monitoring steps (g) and (i). 
     
     
       24. The method of monitoring elongated fuel elements as recited in claim 17 in which the calibration steps (a) to (e) inclusive are performed while said instrument is adjacent said elongated fuel elements and while the nuclear reactor core is in operation, said calibration and monitoring steps comprising comparing the temperature differences between said zones under two conditions one of which conditions occurs while said electrical current is off and the other of which conditions occurs while said electrical current is on.   
     
     
       25. The method of monitoring elongated fuel elements as recited in claim 24 in which the calibration steps (a) to (e) inclusive are performed using several increments of electric power heating. 
     
     
       26. The method of monitoring elongated fuel elements as defined in claim 17 in which said measuring step (e) includes measuring the temperature difference between the "hot" and "cold" junction of a thermocouple, comprising: spacing said "hot" junction from all nearby liquid and solid matter while exposing said "hot" junction to said gamma rays.   
     
     
       27. The method of monitoring elongated fuel elements as defined in claim 26, comprising: positioning said "cold" junction in a bed of solid material and exposing said solid material to said cooling fluid,   whereby said "hot" junction is heated to a temperature above said cold junction by reason of the direct impingement of said gamma rays on said "hot" junction with said cooling fluid having only a secondary effect on the temperature of said "hot" junction.   
     
     
       28. In apparatus for monitoring fuel elements, a nuclear reactor core: a measuring instrument comprising a thermocouple having a "hot" junction and a "cold" junction,   said measuring instrument having a body, said body having an outer wall,   said measuring instrument including means for mounting said "hot" junction inside said body and spaced from any and all liquid and solid material,   said measuring instrument including solid material surrounding said "cold" junction and providing a heat conduction path from said cold junction to said outer wall,   means for passing a cooling fluid along the outer wall of said body, and   means for positioning said body in the path of said gamma rays to thus directly heat said hot junction, whereby the heat from said gamma rays elevates the temperature of the "hot" junction above that of the "cold" junction due to the better thermal contact between the "cold" junction and the cooling fluid than between the "hot" junction and the cooling fluid.